Comments

Richard Rood

The Arctic Oscillation (AO), which for this discussion is equivalent to the North Atlantic Oscillation and the Northern Annular Mode, is the predominant mode of middle and high latitude variability in the northern hemisphere. This is true for the entire year, but the AO's influence is most visible in the winter and early spring. Prior to 1960 the the behavior of the AO is more or less random from one year to the next, which is consistent with climate models. After 1960 there is persistence of patterns from one year to the next, which suggests that something has changed. Models with increasing carbon dioxide better reproduce this persistence, suggesting a relation to greenhouse gas forcing.

The AO is a difficult to predict atmospheric process, which is different from El Nino and La Nina. The El Nino - La Nina cycle is an atmospheric-oceanic oscillation, with some predictability.

The massive reduction of Arctic sea ice and large changes in northern hemisphere snow cover are important changes that occurred after this paper was published and whose impacts need to be evaluated.